How do I use Qt and SDL together?

Question

I am building a physics simulation engine and editor in Windows. I want to build the editor part using Qt and I want to run the engine using SDL with OpenGL.

My first idea was to build the editor using only Qt and share as much code with the engine (the resource manager, the renderer, the maths). But, I would also like to be able to run the simulation inside the editor. This means I also have to share the simulation code which uses SDL threads.

So, my question is this: Is there a way to have an the render OpenGL to a Qt window by using SDL?

I have read on the web that it might be possible to supply SDL with a window handle in which to render. Anybody has experience dong that?

Also, the threaded part of the simulator might pose a problem since it uses SDL threads.

Answer 1

Rendering onto opengl from QT is trivial (and works very well) No direct experience of SDL but there is an example app here about mixing them. http://www.devolution.com/pipermail/sdl/2003-January/051805.html

There is a good article about mixing QT widgewts directly with the opengl here http://doc.trolltech.com/qq/qq26-openglcanvas.html a bit beyond what you strictly need but rather clever!

Answer 2

This is a simplification of what I do in my project. You can use it just like an ordinary widget, but as you need, you can using it's m_Screen object to draw to the SDL surface and it'll show in the widget :)

#include "SDL.h"
#include <QWidget>

class SDLVideo : public QWidget {
    Q_OBJECT

public:
    SDLVideo(QWidget *parent = 0, Qt::WindowFlags f = 0) : QWidget(parent, f), m_Screen(0){
     setAttribute(Qt::WA_PaintOnScreen);
     setUpdatesEnabled(false);

        // Set the new video mode with the new window size
     char variable[64];
     snprintf(variable, sizeof(variable), "SDL_WINDOWID=0x%lx", winId());
     putenv(variable);

     SDL_InitSubSystem(SDL_INIT_VIDEO | SDL_INIT_NOPARACHUTE);

        // initialize default Video
        if((SDL_Init(SDL_INIT_VIDEO) == -1)) {
      std:cerr << "Could not initialize SDL: " << SDL_GetError() << std::endl;
        }

     m_Screen = SDL_SetVideoMode(640, 480, 8, SDL_HWSURFACE | SDL_DOUBLEBUF);
     if (m_Screen == 0) {
      std::cerr << "Couldn't set video mode: " << SDL_GetError() << std::endl;
        }
    }

    virtual ~SDLVideo() {
     if(SDL_WasInit(SDL_INIT_VIDEO) != 0) {
      SDL_QuitSubSystem(SDL_INIT_VIDEO);
      m_Screen = 0;
     }
    }
private:
    SDL_Surface *m_Screen;
};

Hope this helps

Note: It usually makes sense to set both the min and max size of this widget to the SDL surface size.

Answer 3

While you might get it to work like first answer suggest you will likely run into problems due to threading. There is no simple solutions when it comes to threading, and here you would have SDL Qt and OpenGL mainloop interacting. Not fun.

The easiest and sanest solution would be to decouple both parts. So that SDL and Qt run in separate processes and have them use some kind of messaging to communicate (I'd recommend d-bus here ). You can have SDL render into borderless window and your editor sends commands via messages.